Abstract The Illinois basin (Figure 22-1) is a mature producing basin. Major exploration efforts began in the late 1930s, peaked in the 1940s, and have declined generally since. However, high rates of drilling resumed in the early 1980s as oil prices made operations profitable. During the past 30 years, developmental, off-set, and step-out drilling have been staples of oil and gas operations in the Illinois basin. As exploration declined, enhanced oil recovery (EOR) began with waterflooding in the 1950s and continues to the present with waterflooding and other secondary and tertiary methods. More than 4.1 billion barrels of oil have been produced from approximately 1700 fields in Illinois, Indiana, and Kentucky; these fields may contain up to 7000 separate reservoirs. As much as 7.7 billion barrels of oil may remain in place (Mast and Howard, Oil and Gas Production and Recovery Estimates in the Illinois Basin, this volume). An additional 500 million barrels of oil may yet be discovered and produced by primary, secondary, and other EOR methods. The producing reservoirs range from Ordovician through Pennsylvanian in age and are principally carbonates and sandstones. The lower Paleozoic, Ordovician, Silurian, and Devonian reservoirs (Howard, 1967) are for the most part developed in limestones and dolomites. The reservoirs in these carbonates are the results of porosity developed by diagenetic processes of dolomitization and dissolution associated with unconformities. Silurian organic reef buildup and the associated diagenetic features of the reefs produced the Niagaran reservoirs. As yet, the Cambrian and Ordovician sandstones have not proved

Abstract The Granville pay zone produces oil and gas from pelecypod/ gastropod rudstone and grainstone carbonate sediment-piles that accumulated as shoal and beach deposits on a shallow-water carbonate platform in Late Ordovician (Edenian) time. Fresh-water vadose diagenesis produced secondary porosity into which hydrocarbons migrated. There is a predictable trend to these Granville pay zone reservoirs, and other similar reservoirs can be predicted using modern sedimentological techniques.

The Upper Devonian turbidite sequence in the central and southern Appalachian basin reaches 1400 m in thickness and consists of siltstone turbidites interbedded with mudstone, claystone, and shale. Facies and paleocurrent analyses, based on approximately 6100 m of detailed measured section at 35 localities and 700 measurements of directional structures, indicate major differences between the Upper Devonian sequence and submarine fan and ancient flysch deposits. Compared to flysch and submarine fan deposits, the Upper Devonian turbidites are finer grained and thinner bedded, reflecting the smaller size and lower maximum velocity of the turbidity currents. The stratigraphic transition from turbidites to overlying deltaic rocks shows a different succession from that expected for proximal deposits of submarine fans associated with fan channels and canyons. This transition in the Upper Devonian deposits is gradual and marked by upward-thickening and -coarsening sequences of evenly-bedded turbidites and lenticular beds of crossbedded sandstone. Higher energy mass flow deposits such as debris flows, massive and pebbly sandstone, and conglomerates are notably absent. Paleocurrent data are remarkably uniform, both areally and vertically through the stratigraphic section. They indicate a uniform dispersal pattern, transverse to the basin axis, for nearly 600 km along depositional strike. This suggests turbidity currents had multiple point sources along the basin margin and that the paleoslope had little topographic relief on it. Dip-oriented, short-lived turbidite lobes were built and abandoned as the source of turbidity currents shifted. Migrating delta distributaries, instead of fixed feeder channels, probably supplied sediment to the slope.

The Carboniferous rocks of the Eastern Interior basin reach a maximum thickness of 5,700 ft., and the Mississippian and Pennsylvanian subdivisions are separated by a major widespread erosional unconformity. The major subdivisions of the Mississippian are the Kinderhook, Osage, Meramec, and Chester Series, and those of the Pennsylvanian are the McCormick, Kewanee, and McLeansboro Groups. These series and groups constitute the basic stratigraphic slices treated herein. More than 3,200 ft. of Mississippian sediments were deposited in the Eastern Interior basin while thinner deposits accumulated on adjacent arches and uplifts. Biologically and chemically derived sediments dominate the Meramecian rocks of the region, whereas the volume of terrigenous detritus is significant in Kinderhookian and Osagian rocks and constitutes the bulk of Chesterian rocks. The distal marine parts of southward-prograding deltas pulsed into the area during early Kinderhook time. As they waned, carbonate deposition spread from the west and southwest to the east, covering extensive areas by the end of Kinderhook time. The Borden Delta complex prograded into the basin and adjacent eastern areas during Osage time. Siliceous, cherty Osagian rocks accumulated in the southwestern part of the region. Carbonate sedimentation, which was initially restricted to the western areas, again spread eastward and northward as the deltas waned, and reached its maximum extent during the Meramec. Evaporite deposition during mid-Meramec time marked a widespread episode of restricted circulation. The carbonate environment retreated as major delta deposits prograded southward and dominated the central part of the Eastern Interior basin during Chester time. Although generally restricted to southern parts of the region, carbonate accumulation periodically extended over large areas in and beyond the basin during this time. Depositional environments throughout the Mississippian were mostly shallow marine over broad areas, but deeper marine environments were present in the southern parts of the region during the Osage and early Meramec. These shallow seas opened and deepened southward across a broad shelf and connected with the deepening Ouachita Trough. The terrigenous deltaic sediments were transported into the region by the large Michigan River system, which drained eastern parts of the Canadian Shield and northern extensions of the Appalachian Mountain belt. Broad regional uplift marked the close of Mississippian time when the littoral zone retreated southward out of the area. By Early Pennsylvanian time the region was a southwest-inclined coastal plain with a well-developed linear drainage pattern and river valleys as deep as 200 ft. With renewal of subsidence during Early Pennsylvanian, the littoral zone transgressed north toward the Eastern Interior basin resulting in the accumulation of thick sequences of alluvial sands and muds in the pre-existing valleys and in the eventual burial of the unconformity under an apron of alluvial and upper delta-plain sediments. Shallow-marine environments moved into the area, and the long period of southwestward progradation, abandonment, and progradation of a series of deltas began. This Pennsylvanian deltaic sedimentation resulted in deposition of as much as 2,500 ft. of dominantly terrigenous, clastic sediments on a slowly subsiding shallow-water cratonic platform. The Michigan River system passed through the Michigan basin region, depositing mostly fluviatile and upper delta-plain sediments there. The river system with its delta plain and widespread coal-swamp deposits repeatedly prograded into the shallow-marine environments of the Eastern Interior basin, where thin but widespread bioclastic carbonates were accumulating. The cyclothemic character of the Pennsylvanian rocks here is the result of this repeated southwestward regressive progradation of at least 51 delta and subdelta sequences. Erosion of the latest Pennsylvanian sediments prevents us from deducing precisely when Carboniferous deposition ceased. A gradual shift took place from deposition of orthoquartzite sandstones in Late Mississippian–Early Pennsylvanian rocks to subgraywacke sandstones in the Middle and Late Pennsylvanian rocks. The terrigenous sediments of the Eastern Interior basin were derived largely from the tectonic borderlands of the northern part of the Appalachian basin and were transported west and southwestward by the Michigan River system. Throughout Carboniferous time, the Eastern Interior basin was connected across the shallow-marine cratonic platform with the Appalachian geosyncline to the east, the midcontinent basin to the west, and the deepening Ouachita geosyncline to the south.